The food-web of the Cladophora-belt fauna

ANN-MARI JANSSON

Department of Zoology and Ask6 Laboratory, University of Stockholm, Stockholm, Sweden

KURZFASSUNG: Die Nahrungskette der Fauna der Cladophora-Zone. In der Ostsee bildet die Griinalge Cladophora glomerata grolge, giirtelf6rrnige Best~inde in Ufern~ihe. Diese Besfiinde und ihre Fauna sind ftir die/Skologischen Verh~ilmisse in der Ostsee von grof~er Bedeutung. Es wurde daher das Artengefiige dieser Fauna analysiert und Trockengewichtsbestimmungen an C. glomerata durchgefiihrt. Die Produktivit~it dieser Alge betr~igt 3105 m Algenfilamente pro Quadratdezimeter pro Monat (mittleres Trockengewicht eines 1000 mm langen Algenfilamen- tes: 0,25 mg). Pro Quadratdezimeter Felsenoberfl~iche ergeben sich folgende Werte fiir die Hauptorganismengruppen: Trockengewicht ftir Cladophora: 0,78 g, f[ir Diatomeen: 1,25 g; Anzahl Tiere: 12 544 Individuen. In Laboratoriumsexperirnenten konnte nachgewiesen wer- den, dal~ epiphytische Diatomeen der Cladophora ffir die ersten postmarsupialen Stadien des Isopoden Idotea baltica lebensnotwendig und fiir den Gastropoden Hydrobia ulvae sowie ffir Chironomidenlarven zur Erzielung maximaler Wachstumsraten erforderlich sind. Der fiigliche Konsum an Cladophora-Filamenten wurde fiir junge Idotea baltica und Gamrnarus oceanicus an Hand der ausgeschiedenen Faeces ermittelt. Die Nahrungskette innerhalb der Fauna der Cladophora-Zone wird an Hand yon Beispielen illustriert.

I N T R O D U C T I O N

The ecology of animals inhabiting seaweed communities is a rather virgin field. In the Baltic Sea, pertinent studies have thus far been concentrating on the fauna of areas occupied by Fucus vesiculosus. Such areas have proven to be unusually rich in regard to the number of animals species, if one takes the overall reduct ionin species numbers into consideration which is typical for brackish waters. Quantitative studies on the Fucus fauna have been carried out by SEGERSTRKLE (1928, 1944), OHM (1964) and, in (Sresund, by DAHL (1948) and HAGERMAN (1966).

The littoral belts of green algae developing chiefly during summer is botanically well known from the thorough investigations made by LEVRING (1940) and WAERN (1952). There are, however, only very few papers published on the associated micro- fauna. An example is the work by OTTO (1936) on the Enteromorpha-zone. This zone and especially the belt of Cladophora glomerata are important parts of the phytal and exert great influence upon the fauna of the Baltic Sea as a whole. In the first place, the dense vegetation of the filamentous algae Cladophora glomerata provides suitable niches and nourishment for the young stages of some of the most important inverte- brate animals of the Baltic. Secondly, the fauna in the deeper layer of the sea is

Cladophora food-web 575

occasionally direct ly influenced by the photosynthet ic l i t toral zone. Dur ing storms, tufts of Cladophora, heavi ly covered wi th epiphytes, are easily torn a w a y and trans- por ted over great distances. When this green plant mater ial f inally sinks to the sea bottom, it constitutes highly valuable food material .

Q U A N T I T A T I V E M E T H O D S

Dur ing the investigations of the Cladophora-belt on the rocky coast around the Ask6 Laboratory , I have taken quant i ta t ive samples regular ly to study the animal colonizat ion of the growing algae and the changes in the composit ion of animal species

during the season. The animals collected in each sample were careful ly washed out

and the dry weight of the algae was determined. The dry weight of most samples was between 10 and 20 mg. Dur ing each collection the distr ibution of epiphyt ic d ia tom genera was registered and the number of specimens per running mm Cladophora-fila-

Table i

The epiphytes on a 24 mm long filament of Cladophora. The number of specimens has been counted for each mm from the base (section 1) to the apex (section 24) of the filament. (a) Synedra, (b) Epithemia (larger species), (c) Epithernia (smaller species), (d) Licmophora,

(e) Cocconeis, (f) Rhoicosphenia, (g) Navicula

Diatoms size in # Section Bac- Cyano- a b c d e f g Total (mm) teria phycea ca.

100 80-70 25 30 30 20 15

1 + + 144 1 145 2 + + 2 1 89 2 3 126 26 249 3 + + 4 37 1 6 190 32 270 4 + + 1 1 8 2 223 16 251 5 + 3 2 2 20i 21 229 6 + 4 3 1 260 8 276 7 + 1 121 3 125 8 + 1 2 50 1 54 9 2 1 15 3 21

10 1 1 8 1 11 11 3 3 1 7 1 15 12 1 1 2 13 1 3 2 5 3 14 14 2 2 15 1 2 3 16 1 2 3 17 1 2 3 18 1 1 2 19 20 1 1 2 21 22 23 2 2 24 2 2

Total 11 6 285 20 28 1211 120 1681

576 A.-M. JANSSON

ment was counted under a microscope. Table 1 gives an example of the number of epi- phytes occurring on a 24 mm long filament. In this way seasonal variations in the diatom flora have been established. In principle, the results obtained can be regarded represen- tative for the Cladophora-belt of great parts of the Baltic Sea.

ANALYSIS OF ALGAL DRY W E I G H T

Epiphytes often occur in great masses, and consequently greatly affect the dry weight of the Cladophora samples. The dry weight of diatoms has been calculated. Filaments of Cladophora glomerata practically free of epiphytes were measured under the microscope; their average dry weight was 0.25 mg per 1000 mm of filament. This value is only valid if the filaments have a diameter of 30 to 85 ~, which is characteristic for this species in the investigated locality.

By measuring comparable material with a rich diatom growth the total dry weight of the epiphytes was obtained (Table 1). In order to obtain a general value for the weight of different kinds and sizes of diatoms a key for transformation into "diatom- units" (Table 2) was devised. The approximate weight of each diatom-unit has been

Table 2

Key for transforming the different types of diatoms which occur on the green alga Cladophora glomerata into "diatom-units"

Genus Size in ~ "Diatom-unit"

Roicosphenia 15-20 1 Licmopbora 25-35 2 Navicula 15 1 Tabellaria 15-20 1 Synedra ca. 100 5 Cocconeis 30-35 2 Epithemia 20-30 1,5 Epithernia 70-80 4

calculated to be 48 × 10 -7 mg. If the density of diatoms is more than 52 diatom-units per mm of filament, which is often the case, the weight of the epiphytes exceeds that of the Cladophora itself.

T HE TOTAL SURFACE OF ALGAE

A more interesting value than the length of algal filaments is, of course, the total surface available for the animals. Estimations of the species specific total surface have been considered rather impossible to make, especially for tuft-like algae (DAHL 1948, pp. 44--45). WI~SEP, (1951), however, described a method by which samples of different algae can be compared. An "adsorption-coefficient" of each species was determined and considered to be equivalent to the total surface. It is interesting to note that the greatest relative surface was established for a species of Cladophora.

Cladophora food-web 577

In comparing small samples of a single filamentous species I find it more sufficient to determine the dry weight (from which the total length of the filaments can be cal- culated) and to use an approximate value for the filamental diameter in order to obtain the total surface. The principle part of Cladophora cells has a diameter of 50 #; this value is in agreement with diagrams of cell-sizes (presented by S6DeRSTR6M 1963, pp. 44--45), characteristic for Cladophora glomerata. For my practical purposes, I cal- culated the dry weight of a tuft of Cladophora glomerata, having a total surface of 1 dm 2, as 15.9 mg.

T HE P R O D U C T I O N I N T HE CLADOPHORA-BELT

The production in the sea is an extensive subject of which we will not get a complete picture until the mosaic of works on single parts of it are put together. In regard to the production in brackish waters, summer-annual filiform algae are of great importance. During the vegetation period they grow rapidly, imparting considerable quantities of organic nutrients. In the northern part of the Baltic Sea proper the pe- rennial belt of Cladophora glomerata, growing in about 0.5 m depth, reproduces during the middle of July and thus gives rise to a new belt, oflten situated close to the midwaterline. In 1966, zoospores were seen on the 12th of July; on the 20th of July the first new plants were observed on the rock. On the 27th of July, the filaments were about 2 to 3 cm long, and on the 16th of August, when the growth had con- tinued for a month, the algae were about one dm long, and a very dense summer-belt had developed in depths of 1 to 8 cm. This rapid growth rate conforms with that observed by MAsoN (1965) for C. glornerata in farm ponds.

Table 3

Components making up the biomass per dm ~ rock surface area in the Cladophora belt. (Ask/5, August 16, 1966)

Biomass-components/dm ~ rock surface

Cladophora (length) Cladophora (surface area) Cladophora (dry weight) "Diatom-units" Diatoms (dry weight) Macro-animals (number of individuals) Micro-animals (number of individuals)

3105 m 50 dm 2 0,78 g 242X106 1,15 g 44 12500

In a one month old Cladophora-belt, the total dry weight of algae and animals per square unit of rock surface was estimated. All components of the biomass are shown in Table 3. The sampling method was essentially the same as the one used by CI~AVMAN (1955) in the coralline alga zone. A frame enclosing 1 dm ~ was placed on the rock and all algae vegetation inside it was carefully scraped off and bottled in filtered seawater. Two parallel samples were taken; they proved to be quite similar in every detail. The animals were sorted out and the algae washed in distilled water,

578 A.-M. JANSSON

then dried at 100 ° C to weight constancy, which was obtained at about 2 g. The diatom flora of the samples was analysed; it had the composition shown in Table 1. The density of diatom-units was calculated to be 78 per mm of the Cladophora-filament. From the known correlation between algal dry weight and length we get a total sum of 3105 m Cladophora-filaments. Summer alga production in the upper belt of Clado- phora glomerata in the Baltic Sea is consequently about 3 km alga filaments per dm 2 per month. Other brackish waters of the world where seaweed production has been measured are, for example, the southern seas of the U.S.S.R. According to MOROSOVA- VOI)VANITZKAYA (1941) the annual productivity in a Cladophora spp.-vegetation is 275.1 g per m 2 in the Black Sea, which is about the same as in the Baltic Sea.

The mean value of the number of macroscopic animals found was 44. The micro- fauna of the belt was unusually poor; it was calculated to 12 500 specimens per dm ~ of the rock and consisted to 95 °/0 of rotifers. This figure does not include all proto- zoans; only ciliates larger than 100/~ have been counted.

Recalculated for m 2 rock surfaces, our results become comparable with earlier studies on animal density in seaweed communities. COI~MAN (1940) found in Wembury Bay, England, values of 274 320 individuals per m s in a community of Lichina pyg- maea, and 202 735 per m s in Ascophyllum nodosurn with epiphytic Polysiphonia lanosa. From the Mediterranean WlrsrR (1959) reports that the highest density with about 4 million per m s is reached in a Halopteris scoparia community; in the Clado- phora-Enterornorpha zone of the same area the density is 900 000 m 2. CHAVMAN (1955) in the Azores littoral has found 376 800 per m 2 in the Corallina granifera zone. The result obtained in the Cladophora-belt in the Baltic, 1 254 400 per m e, must be con- sidered surprisingly high, considering the fact that this is a brackish area, but the number of species is small; the rotifer Proales reinhardti Em~rNB~RO provides the majority of individuals.

Comparable values from other seaweed communities in the Baltic Sea are not easy to find. The microfauna in the vegetation of Fucus vesiculosus has been dealt with by OI~M (1964). A sample taken in July contained about 15 000 individuals per 500 g algae (wet weight). Nematodes represented the dominating group. According to SEG~r, STR~LZ (1944) the mean value of Fucus per m e in 0.5 to 1 m depth is 9.5 kg in the Tv~irminne area. If this value is used together with OvlM'S material for calculating animal numbers in the Fucus vegetation of the Bay of Kiel, a density of some 310 000 individuals per m s is obtained. This rather low value can, to some extent, be explained by OH~t's semi-quantitative sampling method. Considerably higher density values are reported by I-IAGERMAN (1966) from northern Oresund in a Fucus serratus community. This investigation is also one of the very few concerned with animal density per algal surface. Fucus serratus may harbour up to 46000 non-sessile animals per m e algal surface. The value cannot, however, be directly compared with mine (25 000 animals per m e Cladophora surface) since the surface of the Fucus epiphytes in which most of the animals live has not been taken into consideration.

Cladophora food-web 579

TYPES OF FEEDERS I N T HE F A U N A OF THE CLADOPHORA-BELT

H e r b i v o r e s

The fauna of the Cladophora-belt is to a large extent herbivorous. In Figure 1 some herbivores are shown in their habitats. Some animals eat the filaments of green alga, others feed on the diatoms. On the basis of this information, I considered it inter- esting to know how much food was required by different species. The following

B

O , s m m i

2mrn

r

b

/ / '

i Z~ ,

D

J 5 m m 1 I

Fig. 1: Some herbivores of the Cladophora-belt. (A) Their characteristic positions on the alga, (B) Turbellarian Provortex balticus and rotifer Proales reinhardti, (C) Gastropod Hydrobia ulvae and a chironomid larva, (D) Isopod Idotea baltica and amphipod Garnmarus oceanicus

experiments were carried out to calculate the consumption-rate of some important herbivores. All experiments were performed under natural conditions of light at a constant temperature of 150 C. Food sources were Cladophora, (regarded as being free from diatoms), and cultures of diatoms, chiefly Navicula sp. and Synedra sp.

Browsers, feeding on Cladophora-filaments

Idotea baltica PALLAS At the beginning of July 1966, a number of female Idotea baItica with juveniles

in their brood-pouches were collected from the upper Fucus-region. The specimens

580 A.-M. JANSSON

were kept individually in glass-jars containing seawater and a tuft of Cladophora. The time was noted when the ca. 2 mm long juveniles le~ the brood-pouch.

From one newly released brood 40 specimens were individual ly placed in small glass-bowls filled to a height of 4 ml with filtered seawater. 20 of the test individuals were given a small piece of Cladophora, and the other 20 a few drops of a diatom culture. The faecal pellets emitted in each bowl were counted and removed every 24 hours when new water and food was added. Survival rates were much higher in the group fed with diatoms. 50 % of the animals fed with Cladophora were dead a~er 43 hours, they were apparent ly able though to eat the green alga threads. In the diatom group 50 % of the specimens lived for 15 days.

After the third and fourth postmarsupial moulting there was a sudden reduction in the number of survivors among the diatom fed individuals. This caused me to start an identical experiment using one month old (4 mm long) individuals from another brood. Here the results were reversed: in the diatom fed group 50 % had died within 40 hours, but in the Cladophora fed group all specimens were still alive after more than one month; by this time they had reached a body length of 8 mm.

The results of these two experiments can be summed up as follows: 2 mm long juveniles eat Cladophora-filaments; epiphytic diatoms are necessesary for survival. Later stages of 4 to 8 mm body length have no need of diatoms. On the contrary pure diatom food seems to be detrimental in some way. One month old individuals are able to eat a great amount of Cladophora.

The faecal pellets emitted by animals fed on Cladophora were examined under the microscope, and it was thus possible to estimate the length of the Cladophora- filaments eaten. A photograph of such a pellet with a total filament length of some 8 mm is shown in Figure 2A. From every moulting stage 10 faecal pellets were collected

Table 4

Length of Cladophora filaments consumed within 24 hours by juveniles of Idotea and Gamma- rus, and chironomid larvae respectively

Animals Length Consumption/24 hrs.

Idotea 3 mm 315 mm Idotea 4 mm 641 mm Gamrnarus 3 mm 86 mm Gammarus 4 mm 126 mm Chironomidae 3.5 mm 4 mm

and measured, and the average length of the Cladophora-filament per pellet was calculated as well as the daily consumption (Table 4). The total amount of Cladophora required by one specimen passing each successive postmarsupial stage from 2 to 8 mm body length is about 40 m (Fig. 3).

Observations by DE LA CRUZ (1960) on the feeding act ivi ty of Idotea baltica in the Fucus vegetation have demonstrated that the filiform paraphyses on the Fucus thallus are preferred as food. The average consumption rate of Fucus amounts to one- fifth of the body weight per half an hour. A weed preference has also been observed for Idotea granulosa RATHKE (NAYLOR 1955a). Less than 5 mm long individuals of

Cladophora food-web 581

Fig. 2: Faecal pellets of (A) Idotea battica, (B) Gamrnarscs oceanicus, (C) Chironomid larva (mainly composed of diatom Rhoicosphenia). (D) Part of the gut oi c Chaetogaster limnaei,

containing chironomid larvae and three specimens of ColureIIa colurus (arrows)

582 A.-M. JANSSON

this species are most abundant on small tufted algae such as Cladophora. There are very few studies on the growth of different species of Idotea. NAYLOR'S (1955b) investigation on I. emarginata is the most exhaustive one. Sizes and lengths of the first postmarsupial stages, characteristic for this species, are about the same as in I. baltica.

E 10000 E

< n,t

0 1-

0 L3

t . ) 2000

S T A G E S r n m

Fig. 3: Average food consumption (length of Cladophora filaments in mm) of Idotea baltica (post marsupial stages of different body lengths). The regression-line is eye-fitted

Gammarus oceanicus SEGERSTR~LE In the Cladophora-belt the population density of young gammarids is otten very

high. Since gammarids represent an important prey for many fishes of the Baltic Sea it is of interest to assess their rates of food consumption.

24 about 3 mm long juveniles of Gamrnarus oceanicus were collected and placed in glass-bowls (similar technique as in the Idotea-experiments). Half of the individuals were offered Cladophora; the other half diatoms. Seven weeks later all but 3 indivi- duals were still alive and had moulted about 5 times. No difference in size or activity was noted between the two groups. The mean value of the total length of Cladophora- filaments consumed per day was calculated. A photograph of a faecal pellet emitted by a specimen fed on Cladophora is seen in Figure 2B. Table 4 shows that 3 mm long individuals eat 86 mm filaments and 4 mm long ones 126 mm per day. Compared with Idotea baltica of the same size, Garnmarus oceanicus eats considerably less and grows significantly slower.

Diatom feeders

Chironornidae Larvae of chironomids are usually very numerous in the Cladophora-belt. The

bigger larvae undoubtedly eat green alga threads. It was proved by an experiment that at a body length of 3.5 mm they are able to eat small pieces of Cladophora, but their natural food are diatoms. A photograph of a faecal pellet emitted from a 3.5 mm long specimen just removed from its natural habitat is shown in Figure 2C. A large

Cladophora food-web 583

amount of diatoms, especially Rhoicosphenia, can be seen. The animal was taken from an area where the diatom growth was about the same as under the conditions on which Table 1 is based. Chironomid larvae prefer to sit close to the base of the algae in a dense growth of filaments (Fig. 1).

Hydrobia ulvae PENNANT Gastropods are also typical epiphyte feeders. A poor epiflora of bacteria and

blue-green algae may suffice as food source but ample availabi l i ty of diatoms increases their growth-rate. This was demonstrated in a feeding experiment of the same type as the ones described previously. Small specimens of Hydrobia ulvae fed on diatoms grew more rapidly than those fed on Cladophora, although production rate of faecal pellets was about the same in the two groups. Growth-rates are given in Table 5.

Table 5

Growth rates (length of snail house in mm), of gastropods, Hydrobia ulvae, fed on Clado- phora - filaments without epiphytic diatoms, or cultures of diatom Navicula. Water temper-

ature 150 C; salinity 6.4 %0 S

Duration of Food source Experiment (days) Cladophora Diatoms

0 1 mm 1 mm 22 1.41 mm 1.52 mm 43 1.46 mm 1.85 mm

Proales reinhardti EHRENBERG The first animals immigrating a newly established Cladophora-belt are usually

rotifers. They are pr imari ly represented by Proales reinhardti. Once settled down in the habitat, the population rapidly increases. For example, 2 weeks a~er a Clado- phora-belt had started to grow, more than 200 specimens of P. reinhardti per 10 mg algae were collected.

P. reinhardti feeds on small diatoms. Table 6 gives a list of diatoms found in the guts of 20 specimens. The test material was collected in an area where the epiphytic growth corresponds to the situation referred to in Table 1. The diatom genus Licmo- phora dominates in the guts, although its representatives are rather few in number on the filaments. The microhabitat of P. reinhardti is the peripheri of the Cladophora tuflc where also Licmophora attains its greatest relative abundance. Possibly represen- tatives of this genus are comparatively easy to pick off from the threads, and there- fore more commonly eaten than other diatoms of the same cell size.

D e t r i t u s f e e d e r s

Colurella colurus EHRENBERG For the very numerous rotifer Colurella colurus (50 to 60 # in size) there are

hardly any epiphytic diatoms small enough to be suitable as food. The species feeds on

584 A.-M. JaNSSON

Table 6

Gut contents of 20 adult specimens of the rotatorian Proales reinhardti. The cell sizes of the 5 different diatoms are given in brackets

Specimen Gut contents No. Licmophora Rhoicosphenia Navicuta Tabellaria Epithemia

(30 ~) (20 #) (15 f,) (is #) (25 l,)

1 4 2 1 2 2 1 3 2 4 4 1 1 5 3 2 6 5 2 7 8 2 8 4 6 9 1 1

10 1 11 1 12 15 2 13 8 19 14 7 15 t4 16 1 1 17 2 2 18 1 19 5 1 1 20 5 2

1 1

bacteria and detritus, and for example, on the incompletely digested material in the feacal pellets of Idotea and Gamrnarus.

Tachidius discipes GieSBR~CHT A harpacticoid of regular abundance in the Cladophora-belt is Tachidius discipes.

DARL (1958, p. 87) found this species only in detritus free samples. In my experiments T. discipes reproduced and survived for one year or longer in stagnant water cultures containing samples of Cladophora; the only food source available was slowly decaying parts of this green alga.

C a r n i v o r e s

Aspelta cIydona HARRING & MYERS The Rotator ia are a dominating group in the upper Cladophora-belts of the

Baltic Sea. Most species are diatom feeders but a few are predators. A. clydona periodi- cally found in the samples, for example, preys on other Rotator ia especially on ProaIes reinhardti.

Provortex balticus SCHULZE A permanent predator is the turbellarian Provortex balticus. It catches living

ciliates and rotifers, for example, Proales reinhardti. Its main food sources, however, are diatoms of the genus Epithemia; this was shown by JANSSON (1966).

Ctadophora food-web 585

Chaetogaster limnaei YON BAER The greatest predator among the animals in the Cladophora-belt is probably

Chaetogaster lirnnaei. Typical prey organisms are Tachidius discipes, larvae of Chiro- nomidae, and Colurella colurus (Figs. 2 D; 4). As C. limnaei is periodically rather common in the belt, this predator might affect the species composition of the fauna.

Fig. 4: Anus region of Chaetogaster limnaei; beneath a freshly emitted Tachidius discipes

S c a v e n g e r s

Feeding on dead animals has by many authors been considered characteristic for the adults of both Idotea and Garnmarus. In the Cladophora-belt I do not think that this type of nourishment is of any importance for the young Idotea. Growlng-up Garnrnarus, on the other hand, may be scavengers when the opportunity arises. In an exposed Ctadophora-belt, however, where Garnrnarus oceanicus is found, most dead animals are quickly washed away.

586 A.-M. JANssoN

S u s p e n s i o n - f e e d e r s

Cardium larnarcki REEVr This bivalve metamorphoses in great numbers in the Cladophora-belt. The young

animals sit in the Cladophora tui%s feeding on suspended material which is con- centrated in the filamentous vegetation.

CONCLUSIONS

As already pointed out by several authors, the great total surface of the filamen- tous green alga Cladophora makes it a suitable micro-habitat for dense animal coloni- sation. The attractiveness of its surface area is highly increased by epiphytic diatoms. The importance of these epiphytes as a food source for the fauna associated with Cladophora is outstanding. For the young stages of some species the diatoms are even necessary for survival. Later stages of these species do no longer depend on diatoms as a food source and may indeed live in quite different habitats.

A characteristic food web exists in the Cladophora-belts; a part of this food web is schematically illustrated in Figure 5.

THE FOODWEB IN THE CLADOPHORA- BELT

T \ C LADOPHORA

............ l DIATOMS .... I DETRITUS

Fig. 5: Some important components of the food-web in the Cladophora-bett

Typical food chains are, for example, diatoms ~ Proales reinhardti ~ Provotex balticus and Cladophora + diatoms -+ chironomid larvae ~ Chaetogaster limnaei.

In the production of organic matter in the sea, Idotea and Gammarus are of great importance as fish food; their juveniles, therefore, must be considered the most im- portant components of the food web in the Cladophora-belts.

Cladophora food-web 587

SUMMARY

1. The species composition of the fauna in the Cladophora glomerata belts of the Baltic sea has been studied quantitatively.

2. Examples are presented to illustrate composition and density of the epiphytic diatom flora.

3. The dry weight of Cladophora glomerata was determined with the aid of length measurements under a microscope. It amounts to 0.25 mg per 1000 mm length of alga filaments, and to 15.9 mg for a tui°c with a total surface area of 1 dm 2.

4. The productivity of Cladophora glornerata was calculated. I t amounts to 3105 m alga filaments per dm 2 per month.

5. The following biomasses per dm 2 rock surface have been estimated: Cladophora (dry weight): 0.78 g, diatoms (dry weight): 1.15 g, animals (number of indivi- duals): 12 544.

6. The fauna of the Cladophora-belt has been subdivided according to feeding types. 7. Some herbivores have been studied in laboratory experiments. The epiphytic

diatoms are important for survival of the first postmarsupial stages of the isopod Idotea baltica as well as for maximum growth rates of the gastropod Hydrobia ulvae and chironomid larvae.

8. The daily consumption of Cladophora-filaments has been determined for juveniles of the isopod Idotea baltica and the amphipod Garnmarus oceanicus by micro- scope examinations of faecal pellets.

9. The diatom diet of the rotifer Proales reinhardti has been listed according to genera or species. Among the animals living in the Cladophora-belt, P. rein- hardti attains the highest numbers of individuals.

10. Examples of food-chains within the Cladophora community are presented.

L ITERATURE CITED

CaAPMAN, G., 1955. Aspects of the fauna and flora of the Azores. 6. The density of animal life in the coralline alga zone. Ann. Mag. nat. Hist. 95, 801-805.

COLMAN, J., 1940. On the faunas inhabiting intertidal seaweeds, J. mar. biol. Ass. U. K. 24, 129-183.

CRuz, A. A. DE LA, 1960. Observations on the feeding activity of the isopod, Idotea baltica (PALLAS). Biol. Bull. mar. biol. Lab., Woods Hole 110, 312.

DAHL, E., 1948. On the smaller arthropoda of marine algae, especially in polyhaline waters off the Swedish West Coast. In: Undersfkningar 6ver Oresund 35, 1-193.

HAG~RMAN, L., 1966. The macro- and microfauna associated with Fucus serratus L., with some ecological remarks. Ophelia 3, 1-42.

JANSSON, A. M., 1966. Diatoms and microfauna-producers and consumers in the Cladophora belt. Ver6ff. Inst. Meeresforsch. Bremerh. (Sonderbd) 2, 281-288.

LEVRING, T., 1940. Studien ~iber die Algenvegetation yon Blekinge, Siidschweden. Lund, Diss., 178 pp.

MASON, CH. P., 1965. Ecology of CIadophora in farm ponds. Ecology 46, 421-429. MOROSOXrA-VoDxANITZI~AYA, N., 1941. Materials bearing on the vegetative productivity of

the Black Sea. Tr[udy] z[ooL] I[nst., Leningr.] 7, 2 (E[ngl.] s[ummary]). (Cited according to Zenkevitch, L.: Biology of the seas of the U.S.S.R. Transl. from the Russ. London: Allen & Unwin 1963. p. 875.)

588 A.-M. JANSSON

NAYLOR, ]~., 1955a. The life cycle of the isopod Idotea emarginata FABRICIUS. J. Anirn. Ecol. 2 4 , 270-281.

- - 1955b. The ecological distribution of British species of Idotea, Isopoda. J. Anita. Ecol. 24, 255-269.

OHM, G., 1964. Die Besiedlung der Fucus-Zone der Kieler Bucht und der westlichen Ostsee unter besonderer Ber/.icksichtigung der Mikrofauna. Kieler Meeres/orsch. 20, 30-64.

OTTO, G., 1936. Die Fauna der Enteromorphazone der Kieler Bucht. Kieler Meeresforsch. 1, 1-48.

SEOERSTR~L~, S. G., 1928. Quantitative Studien iiber den Tierbestand der Fucus-Vegetation in den Sch~iren yon Pellinge (an der Siidkiiste Finnlands). Commentat. biol. $ (2), 1-14.

- - 1944. Weitere Studien iiber die Tierwelt der Fucus-Vegetation an der Siidkiiste Finnlands. Commentat. biol. 9 (4), 1-28.

S6DERSTR6M, J., 1963. Studies in Cladophora. Botanica gothoburg. 1, 1-147. WAEt~N, M., 1952. Rocky shore algae in the ESregrund archipelago. Acta phytogeogr. Suec. 30,

1-298. WIESER, W., 1951. 1]ber die quantitative Bestimmung der algenbewohnenden Mikrofauna

felsiger Meereskiisten. Oikos 3, 124-131. - - 1959. Zur Ukologie der Fauna mariner Algen mit besonderer Beriicksichtigung des Mittel-

meeres. Int. Revue ges. HydrobioL 44, 137-180.